beagle

Beagle 0.2.1 is out. Beagle is a very useful desktop search engine for GNOME (but you can use it with KDE, too, or even without any desktop environment like I do). Despite the low version number it is already quite stable and usable, and has lots of features.

Yes, Google Desktop Search and Spotlight do the same thing, but neither is available for Linux, and neither of them is Free Software. They can "phone home" without telling you and do other funny things. With Beagle you can easily check what it does (use the source, Luke!).